1. Field of the Invention
This invention pertains to an impact and penetration resistant laminate suitable for use in hard armor.
2. Description of Related Art
U.S. Pat. No. 4,309,487 to Holmes describes a laminated armor structure consisting of one or more plies of unidirectionally oriented polyethylene film or fibers which are positioned so that the lines of orientation of adjacent units are at angles to each other. Bonding of the plies is achieved solely through the application of heat and pressure to the composite of positioned plies.
U.S. Pat. No. 7,972,679 to Lyons et al discloses a ballistic-resistant molded article having a sandwich-type structure including two outer portions of a first high modulus material surrounding an inner portion of a second high modulus material. The outer portions are comprised of a plurality of interleaved layers of adhesive coated cross-plied non-fibrous ultra-high molecular weight polyethylene tape. The inner portion is comprised of a plurality of interleaved layers of high modulus cross-plied fibers embedded in resin. The stack of interleaved layers is compressed at high temperature and pressure to form a hybrid sandwich ballistic-resistant molded article that includes a mix of high modulus materials. It has been found that ballistic resistance is higher for the hybrid structure than for a monolithic structure of comparable areal density.
U.S. Pat. No. 7,976,932 to Lyons et al teaches a ballistic resistant panel including a strike face portion and a backing portion. The strike face portion includes a plurality of interleaved layers of non-fibrous ultra-high molecular weight polyethylene tape. The backing portion includes a plurality of interleaved layers of cross-plied fibers of ultra-high molecular weight polyethylene. The entire stack of interleaved layers is compressed at high temperature and pressure to form a ballistic resistant panel having a strike face on one side. It was been found that ballistic resistance increases as the weight ratio of the strike face portion with respect to the backing portion decreases. A composite panel having a strike face of Tensylon® tape with at most 40% of the total weight of the panel exhibits improved ballistic resistance properties as compared to a monolithic structure of strictly interleaved layers of cross-plied high modulus fibers.
U.S. Pat. No. 8,197,935 to Bovenschen at al discloses a ballistic-resistant moulded article having a compressed stack of sheets including reinforcing elongate bodies, where at least some of the elongate bodies are polyethylene elongate bodies that have a weight average molecular weight of at least 100,000 gram/mole and a Mw/Mn ratio of at most 6.
U.S. Pat. No. 7,993,715 to Geva at al relates to polyethylene material that has a plurality of unidirectionally oriented polyethylene layers cross-plied and compressed at an angle to one another, each polyethylene layer composed of ultra-high molecular weight polyethylene and essentially devoid of resin. The invention further relates to ballistic resistant articles that include or incorporate the inventive polyethylene material and to methods of preparing the material and articles incorporating same.
Ultra-high molecular weight polyethylene continuous filaments or fibers may be produced by a gel spinning process. A plurality of such filaments may then be combined to form a yarn. Such multi-filament yarns are available from Honeywell International Inc. or DSM under the tradenames SPECTRA and DYNEEMA respectively. Exemplary patent publications describing this technology are US 2011/0266710, US 2011/083415, US 2006/051577 and U.S. Pat. No. 6,689,412.
The use of an adhesive to provide a bond between two sheets of UHMWPE film is known in the art. However, these adhesive matrices melt between the UHMWPE layers and therefore tend to act as lubricants, making a multi-layer assemblage of sheet and adhesive unstable under manufacturing conditions that require high pressure. When under pressure, the sheets move relative to each other in order to relieve small stress imbalances. If the UHMWPE sheets shift, the finished article's “dimensional stability”, that is to say its shape and fitness for use are potentially compromised. Further, sheet slippage can also present a safety issue during production. The dimensional stability of a multi-layer sheet and adhesive assemblage reduces further as the thickness of the assemblage increases. In this context, dimensional stability is a comparison of the shape of the article after molding compared to the shape before molding. Ideally, the two shapes should be the same with no lateral movement. There remains a need therefore to provide a multi-layer sheet and adhesive assemblage in which adjacent sheets will not move relative to each other during a pressing or lamination process.